1. Field of the Invention
The present invention relates to speaker units mounted in small electronic devices such as personal computers and portable telephones.
2. Description of Related Art
As shown in FIG. 5, a conventional speaker unit includes a speaker device accommodated in a frame 21 made of a synthetic resin. The speaker device includes a magnet 6, and a pole piece 7 and a yoke 5 disposed on opposite polar faces of the magnet 6. A coil piece 91 of a voice coil 9 is disposed in a magnetic field formed between the pole piece 7 and the yoke 5.
The frame 21 has openings 24, 25 at opposite ends in an acoustic wave generating direction of the speaker device. One of the openings 25 is closed with a vibration wall 20. The vibration wall 20 includes a ring-shaped diaphragm 3, and a dust proof 4 that closes an opening of the diaphragm 3 and is coupled to the diaphragm 3. The diaphragm 3 is coupled at its inner periphery to a coil core 92 of the voice coil 9. The diaphragm 3 is pinched at its outer periphery between a peripheral edge of the one opening 25 of the frame 21 and a ring-shaped frame body 2, and fixed to the frame 21.
The other opening 24 of the frame 21 is closed with the yoke 5. The yoke 5 is in the form of a bottomed cylinder having integrally formed bottom 50 and cylinder 51, and fixed with an outer surface of the cylinder 51 of the yoke 5 being in close contact with an inner surface of the other opening 24 of the frame 21. In the frame 21, an end face 53 of the cylinder 51 of the yoke 5 is exposed from the other opening 24. The end face 53 is formed flush with an inner surface 27 of the frame 21.
A pair of lead wires 90, 90 for powering the voice coil 9 extend from the voice coil 9. The lead wires 90, 90 are passed between the end face 53 of the cylinder 51 of the yoke 5 and the vibration wall 20, and led out of the frame 21 (see, for example, JP 9-65485, A).
When the voice coil 9 is powered, the voice coil 9 vibrates as shown in FIG. 6. Its amplitude becomes larger with increase of power applied to the voice coil 9. When a prescribed maximum power is applied to the voice coil 9, the voice coil 9 vibrates at a prescribed maximum amplitude corresponding to the prescribed maximum power. At this time, the voice coil 9 comes closest to the bottom 50 of the yoke 5, so that the lead wires 90, 90 will accordingly come closest to the end face 53 of the yoke 5.
If the power supplied to the voice coil 9 contains noise and thereby exceeds the prescribed maximum power, the voice coil 9 vibrates at an amplitude exceeding the prescribed maximum amplitude, and brings the lead wires 90, 90 into contact with the end face 53 of the yoke 5. This can cause short circuit between the yoke 5 and the voice coil 9 to thereby damage the speaker unit or an externally connected drive amplifier.
Accordingly, a dimension more than a maximum displacement magnitude (saturation displacement magnitude), at which the amplitude of the voice coil 9 stops increasing even if the power applied to the voice coil 9 increases, is secured for a distance W′ between the lead wires 90, 90 and the end face 53 of the yoke 5 in a non-powered state of the voice coil 9, such that the lead wires 90, 90 will not collide with the end face 53 of the yoke 5 even if the voice coil 9 vibrates at an amplitude exceeding the prescribed maximum amplitude.
In recent years, the amplitude of the voice coil 9 has been increasing in order to provide high sound quality and high sound pressure. On the other hand, thinner speaker units have been desired as electronic devices have been made thinner. However, with the conventional speaker unit shown in FIG. 5, if a distance D′ from the outer surface of the bottom 50 of the yoke 5 to the top end of the frame body 2 is made thinner with the cylinder 51 of the yoke 5 and the coil core 92 of the voice coil 9 having smaller height dimensions, the distance between the lead wires 90, 90 and the yoke 5 is narrower as shown in FIG. 7. This has caused a problem in that, if the voice coil 9 vibrates at an amplitude exceeding the prescribed maximum amplitude, the lead wires 90, 90 collide with the end face 53 of the yoke 5 to cause short circuit between the yoke 5 and the voice coil 9. On the other hand, there has been another problem in that a smaller prescribed maximum amplitude of the voice coil 9 could not provide high sound quality and high sound pressure.